libsframe: adjust an incorrect check in flip_sframe

When sframe_encoder_write needs to flip the buffer containing the SFrame
section before writing, it is not necessary that the SFrame FDES are in
the order of their sfde_func_start_fre_off.  On the contrary, SFrame
FDEs will be sorted in the order of their start address.  So, remove
this incorrect assumption which is basically assuming that the last
sfde_func_start_fre_off seen will help determine the end of the flipped
buffer.

The function now keeps track of the bytes_flipped and then compares it with
the expected value.  Also, added two more checks at appropriate places:
 - check that the SFrame FDE read is within bounds
 - check that the SFrame FRE read is within bounds

libsframe/

	* sframe.c (flip_sframe): Adjust an incorrect check.
	Add other checks to ensure reads are within the buffer size.